Electrical stimulation and response monitoring is implemented for analyzing a medium such as biological tissues. For avoiding that the medium is altered by any DC voltage component which may appear between electrodes applied to this medium, and then may cause unwanted electrolysis processes within the medium, each stimulation sequence is comprised of a first time period for injecting electrical charges into the medium, and a second time period for draining from the medium a charge amount which equals that of the charges injected. The second time period of the stimulation immediately follows the first one. For ensuring that the electrical charge amounts which are injected into and drained from the medium are equal, the electrical stimulation is performed through a DC-blocking capacitor.
Commonly, a set of electrodes is built as a braid and arranged so that all the electrodes are simultaneously in contact with the medium. Each one of these electrodes is intended to produce a stimulation sequence as described above, or to collect an electrical response from the medium. For this reason, these electrodes are called stimulation or sensing electrodes. It is then essential that no cross-coupling occurs between different ones of the electrodes during stimulation and also during response monitoring, which would be due to electronic circuits connected to the electrodes.
Each electrode may be either a stimulation electrode or a sensing electrode, or may have alternately the stimulation function or the sensing function depending on a stimulation channel or a sensing channel being currently connected to this electrode. Each electrode is thus connected to a stimulation or sensing channel which is dedicated to this electrode through a DC-blocking capacitor, separately from the other electrodes. Hence a set of capacitors is to be provided for the whole set of electrodes, such that one capacitor is connected electrically between one of the electrodes and the corresponding stimulation or sensing channel.
Up to now, all the DC-blocking capacitors of an electrical stimulation and monitoring device which has multiple stimulation or sensing electrodes are provided as discrete components. These discrete capacitors are mounted on a substrate such as a printed circuit board or a ceramic substrate.
Then it would be cost-effective and allow denser integration to provide all the DC-blocking capacitors which are necessary for all the stimulation or sensing electrodes in the form of an integrated circuit which is produced from one semiconductor substrate. Such electrical stimulation and monitoring device would then comprise:                a semiconductor substrate of a first conduction type, which is provided with a set of separated wells all having a second conduction type, the first and second conduction types being opposite so that each well forms a respective embedded diode at a boundary between this well and a bulk portion of the substrate having the first conductivity type;        a set of capacitor structures which are each accommodated within one of the wells separately from the other capacitor structures, each capacitor structure having a first electrode, a second electrode and a layer portion of an electrically insulating material, the first electrode being formed by the well which is dedicated to this capacitor structure, and the layer portion of insulating material being arranged between the first and second electrodes;        a set of stimulation or sensing electrodes which are each connected to the first electrode of one of the capacitor structures; and        a set of stimulation or sensing channels which are each connected to the second electrode of one of the capacitor structures, and each comprising a current source for injecting a stimulation current into one of the stimulation or sensing electrodes through one of the capacitor structures which is connected serially between the stimulation or sensing channel and the stimulation or sensing electrode, or comprising a sensing circuit for allowing monitoring of a voltage response collected by one of the stimulation or sensing electrodes, and detected by the sensing circuit through the capacitor structure which is connected serially between the stimulation or sensing electrode and the stimulation or sensing channel.        
According to such design, the device is comprised of a set of signal paths each comprised of one stimulation or sensing electrode, one capacitor structure and one stimulation or sensing channel which are connected serially in this order. The capacitor structure has the DC-blocking function mentioned earlier. Electrical insulation between the capacitor structures, within the integrated circuit, is provided by the diodes which are formed at the boundary between each well which accommodates one of the capacitor structures and the bulk portion of the substrate. But because such well diodes are not actually perfect, they have junction capacitors and leakage currents which cause cross-couplings between separate ones of the signal paths. Then stimulation which is intended to be applied to the analyzed medium though a selected one of the electrodes also produces unwanted stimulation through another one of the electrodes. This results in unwanted voltage build-up across some parts of the medium.
Generally, after one stimulation sequence has been applied to the medium through one of the electrodes, a blanking time is necessary before starting sensing the response from the medium, because a residual voltage which has been caused within the medium by the stimulation sequence needs to relax. Indeed such residual voltage may saturate an amplifier which is implemented within the sensing channels, thus making it impossible to sense the true response of the medium which corresponds to biological activity before the residual voltage has relaxed. Blanking time thus limits the response which can be collected, and also limits the number of stimulation and sensing cycles which can be performed within a fixed duration.
When the device is used for therapy purpose, a number of successive stimulation sequences are to be applied at a frequency which is also limited by the blanking time. Indeed, the medium has to relax after each stimulation sequence for avoiding that the residual voltages which are caused by the sequences accumulate and produce an overall voltage runaway within the medium.
Therefore, it is an issue for both uses of electrical stimulation and monitoring devices to shorten the blanking time.
Additionally, if a failure such as dielectric breakdown occurs in one of the capacitor structures of the above device which is based on a semiconductor substrate for hosting the DC-blocking capacitors, a resistive conduction path which passes through the medium appears. Then, an unwanted DC current which would flow along such path can damage the medium. Therefore, it is desirable to have a security dielectric isolation in addition to the capacitor structure within each loop formed by one of the signal paths and a reference voltage branch which leads to a reference electrode also applied to the medium.